The air cleaner of a motorcycle is typically placed in a space generally located under the rear end of a fuel tank and a seat as disclosed in Japanese Utility Model Laid Out Publication No. 59-30790 and Japanese Patent Laid Out Publication No. 60-185685.
The casing of an air cleaner is required to have a certain large volume not only for giving a sufficient surface area to the filter element which is to be installed in the casing but also, in the case of a multi-cylinder engine, for attenuating intake air noise by preventing the interference between the pulsating flows of intake air of different engine cylinders.
However, when one attempts to increased the volume of the casing of an air cleaner by extending it upward or sideways, the height of the seat has to be increased or the rider will experience a difficulty in gripping the motorcycle body between his knees as the case may be, causing considerable discomfort to the rider in either case. If the casing of the air cleaner is extended rearward, the wheel base of the motorcycle must be increased for accommodating the engine intake system, and the maneuverability of the motorcycle will be impaired.